The present invention relates to innovative single-component polyurethane systems based on water-soluble polyurethanes or their aqueous solutions, to a process for preparing them, and to the use.
Aqueous binders based on polyurethane dispersions are well-established prior art and are described for example in Houben-Weyl, Methoden der organischen Chemie, 4. ed. volume E 20, p. 1659 (1987), J. W. Rosthauser, K. Nachtkamp in “Advances in Urethane Science and Technology”, K. C. Frisch and D. Klempner, Editors, Vol. 10, pp. 121-162 (1987) or D. Dietrich, K. Uhlig in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 21, p. 677 (1992).
In this context, amino alcohols are synthesis components often described. Thus DE-A 4237965 describes aqueous polyurethane dispersions which are obtained by reaction of di- or polyisocyanates, hydrophobic polyols containing dimer diol and hydrophilicizing compounds. The examples describe dispersion with fairly low solids contents of 25% to 40% by weight, but the use of hydrophobic diols containing dimer diols, which is essential to the invention, severely limits the variability of such products. Any possible and described reaction of the isocyanate-functional intermediate with amino alcohols prior to dispersing is not an essential part of the invention, since trials can be used as well and also since direct dispersing and reaction with water is possible.
DE-A 4337961 describes aqueous coating materials comprising water-dilutable polyurethane resin, preparable by reaction of polyisocyanate, hydrophilicizing components, polyester polyols and/or polyether polyols if desired, and low molecular weight polyols if desired, to give an isocyanate-functional prepolymer having an acid number of 18 to 70 mg KOH/g, some of the isocyanate groups being reacted in a further step with blocking agent, with addition, if desired, of further polyisocyanate and subsequent reaction with compounds having at least one primary or secondary amino group and at least one hydroxyl group. This gives polyurethane dispersions which are self-crosslinking under baking conditions (at 160° C. in the examples), in other words dispersions in which one molecular contains not only hydroxyl groups but also blocked isocyanate groups, having relatively low solids contents (37% to 42% by weight in the examples) for baking enamels, especially for baking surfacers in automotive finishing. In view of the low amounts of amino alcohols incorporated, the urea group contents, the functionalities relative to hydroxyl groups, and the hydroxyl group contents are low.
DE-A 10214028 describes polyurethanes for water-dilutable surfacer compositions in automotive finishing, have a solids content of greater than 50% by weight which under baking conditions of from 140° C. meet the requirements concerning stone-chip resistance and exhibit overbake stability. It is described how such high solids contents are not achievable with water-dispersible polyurethanes containing neutralized dimethylolpropionic acid as a hydrophilicizing agent. The water-dilutable polyurethanes of the invention, having at least two free hydroxyl groups, are obtained by reaction of alkanol amines with an NCO compound to give a hydroxy-functional intermediate, followed by the addition reaction of a cyclic carboxylic anhydride with the hydroxyl groups, to form ester linkages. The carboxyl and/or carboxylate groups necessary for the dispersing of the polyurethane are therefore incorporated into the polymer via an acid anhydride. This type of acidification via anhydrides leads to the incorporation of the hydrophilicizing compound by way of monoester bonds. It is known that structures of this kind are sensitive to hydrolysis, and therefore the durability of such dispersions is very limited. In the examples, polyurethane dispersions are obtained which have solids contents of 43% to 45% by weight. High functionalities and high hydroxyl group contents are not achievable by this route, since some of the hydroxyl groups are consumed by the reaction with the acid anhydride.
DE-A 10147546 describes self-crosslinking polyurethanes in organic solution, obtained by reaction of special aliphatic-aromatic polyester, partially blocked polyisocyanate and a compound having at least two isocyanate-reactive groups, such as an amino alcohol, for example, which, when used as base coat material, are said to have advantageous properties and which possess good CAB compatibility. The polyurethanes are in solution in relatively large amounts of organic solvents and therefore no longer meet the present-day requirements in relation to emissions reduction.
DE-A 19849207 describes water-thinnable binder compositions comprising water-dilutable polyurethane urea paste resins and polyether polyols for the formulation of pigments paste for incorporation into aqueous coating compositions. The water-dilutable polyurethane urea paste resins described are reaction products of polyol, hydrophilicizing component, polyisocyanate and hydroxy amine and additionally comprise a further polyether polyol component. Suitable hydroxy-functional monoamines are amines with primary amino groups and amines with secondary amino groups. According to the disclosure, polyurethane dispersions are obtained therefrom that contain preferably organic solvents and have solids contents of up to 50%, preferably up to 42%, by weight. The polyurethane dispersions prepared in the examples have solids contents of 30% to 35% by weight and also NMP contents of approximately 6% by weight. These products therefore no longer satisfy modern-day requirements in relation to solvent content and high solids content, and, moreover, the mandatory use of polyether polyols restricts the possible uses to applications in which light fastness and weathering stability are of minor importance.
Despite the fact that the prior art concerning aqueous polyurethane dispersions is very extensive, there continues to be a great demand for improved aqueous products. Required in particular are low to zero emissions, high solids contents, high processing reliability and robustness to external influences, as for example to fluctuating levels of atmospheric humidity or low storage temperatures, high coat thicknesses achievable without defects, stability to hydrolysis, excellent film-mechanical properties, and frequently, in addition, high crosslinking densities and/or high functionalities.
A problem affecting disperse systems such as those of the kind identified above is the fact that, for actual film formation during the coating operation, the coalescence and filming of the disperse polymer particles must take place in such a way as to produce a homogeneous, optically flawless film. Owing to the complexity of the operation, this is significantly more difficult and affected by error than in a case of systems in which the film-forming polymer is in a dissolved state.
In contrast to solutions of polyurethanes in organic solvents, high-quality polyurethane solutions in water have been hitherto unknown.
It was an object of the present invention, then, to provide aqueous single-component polyurethane systems having improved film-forming properties, which meet the abovementioned requirements and which are suitable for optically flawless coatings with an advantageous profile of properties, for example, for use in enameling.